OBJECTIVE: To determine the relationship between maternal ABO blood group and risk of adverse pregnancy outcomes.STUDY DESIGN: Data on ABO phenotypes and pregnancy outcomes were collected from medical records of 5320 singleton pregnant women who had ABO blood testing and follow-up care until delivery in our institution. Adverse pregnancy outcomes that were studied in relation to maternal blood group included preeclampsia, gestational diabetes mellitus (GDM), preterm delivery, low birth weight (LBW) and small for gestational age (SGA) infants.RESULT: Out of 5320 women, 350 (6.6%), 333 (6.3%) and 543 (10.2%) women were diagnosed with preeclampsia, GDM and preterm delivery, respectively. LBW and SGA were, respectively, observed in 394 (7.4%) and 178 (3.3%) infants. By uni- and multivariable analyses, women with A or AB blood types, but not B, were found at increased risk of preeclampsia compared with O type individuals; adjusted relative risks were 1.7 (95% confidence interval (CI), 1.3 to 2.3; P=0.001) for A phenotype and 1.7 (95% CI, 1.1 to 2.6; P=0.01) for AB phenotype. There were no significant relationships between blood types and GDM, preterm delivery, LBW or SGA.CONCLUSION: Maternal ABO blood group was associated with the risk of preeclampsia, but not with GDM, preterm delivery, LBW or SGA.